1. Field of the Invention
The invention relates to a light emitting diode (LED), and more particularly to a LED having a special structure and fabrication method thereof.
Light emitting diodes (LEDs) are well known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays.
2. Brief Discussion of the Related Art
FIG. 1 shows a conventional LED structure, comprising substrate 10, LED chip 20, n-type ohmic contact electrode 30 and p-type ohmic contact electrode 40. The LED chip 20 comprises n-type semiconductor 21, active layer 22 and p-type semiconductor 23. The n-type and p-type ohmic contact electrodes 30 and 40 contact the n-type and p-type semiconductor layers 21 and 23 electrically. The light emitting surface is the top surface of the LED chip 20, as shown in FIG. 1. In this LED structure, n-type and p-type ohmic contact electrodes 30 and 40 are on the LED chip 20 top surface, such that emitted light is covered by the n-type and p-type ohmic contact electrodes 30 and 40, resulting in decreased LED luminescent efficiency.
Furthermore, the n-type contact electrodes 30 can be disposed on the substrate 10 bottom surface, as shown in FIG. 2. However, the emitting light is still covered by the p-type ohmic contact electrode 40, and the substrate 10 must be conductive material such that the n type semiconductor layer 21 is in electrical contact with the p type semiconductor layer 30.
The above-mentioned problem can be addressed by a flip chip, as shown in FIG. 3. The flip chip is an inverted FIG. 1 LED structure, wherein the n-type and p-type ohmic contact electrodes 30 and 40 are in the bottom position and the substrate 10, acting as light emitting surface is in the top position. As the substrate is transparent and without any shelter thereon, the p type reflective layer reflects light upward, thus improving luminescent efficiency of the LED.
Additionally, the extraction efficiency of semiconductor light emitting devices (LEDs) is limited by the high contrast between the optical refractive indices of semiconductor materials (about 2.2-3.6) and that of the surrounding media, typically air (about 1.0) or optically transparent epoxy (about 1.5). This large difference, in refractive indices causes photons within the device to have a high probability of being totally-internally-reflected when impinging on interfaces between the semiconductor and the ambient media. As the conventional LED is typically cubic, which surfaces are parallel to one another, lights with reflective angle larger than the critical angle would totally-internally-reflect within the LED and were absorbed by the active region or defects rather than refract out, thus decreasing luminescent efficiency of the LED.
To decrease the total-internal-reflection problem, a truncated inverted pyramid LED (TIP LED) and a surface texture structure has seem provided. To solve above problem, HP developed a truncated inverted pyramid LED (TIP LED) by cutting sides of the LED chip to be non-parallel, and thus light emits out of the chips more efficiently. Such a TIP LED is disclosed in M. R. Krames et al., Appli. Phys. Lett. 75(16),2365, 1999, U.S. Pat. No. 6,229,160 and U.S. Pat. No. 6,323,063. However, this technique can only apply for easy-processing (e.g. cutting) materials, such as AlGaInP/GaP. The most popular white LED is GaN LED with sapphire substrate, which is too hard to cut, however. Accordingly, the above method cannot apply to GaN white LED for improving luminescent efficiency. CREE employs SiC substrate, which is easier to cut than sapphire substrate, to form a TIP LED, as disclosed in Compound Semiconductor, 7(1), 7, 2001. However, GaN and SiC have lattice-mismatch problems, and SiC has increased absorption coefficient in short wavelengths, which decreases luminescent efficiency.
Surface texture structures have the same problem as TIP LED, and texturing steps may dip through the substrate to decrease LED performance. In U.S. Pat. No. 6,133,589, the epitaxy is grown on a textured surface of a sapphire or AlGaInP substrate. When light reaches the textured surface, it changes direction such that the emission efficiency increases. However, the sapphire substrate surface is difficult to texture, and although the AlGaInP substrate surface can be textured, the epitaxy quality deteriorates.
Furthermore, in U.S. Pat. No. 6,258,618, the textured-surface is formed on the p type semiconductor layer. However, this makes the surface of the p type semiconductor layer uneven, thus increasing the resistance. In addition, the p type semiconductor layer is typically not thick enough to avoid digging through when texturing the same. As a result, the electron-hole surface recombination in the luminescent zone of the luminescent layer decreases, negatively affecting the performance. In addition, in Compound Semiconductor January 2002 to Schmid and Windisch et al., an electrode is positioned between the carrier layer and semiconductor layer by wafer bonding, and a light-emitting surface without electrode is formed by the lift-off method. The light-emitting surface is than textured to increase luminescent efficiency. Although this method can solve the high resistance problem due to uneven surface of the p type semiconductor layer, the digging through issue is still a concern when texturing. An addition support layer is therefore needed, which complicates the fabrication process, however. Accordingly, it is desirable to solve the above problems and provide a LED and fabrication method thereof with improved luminescent efficiency.